[Qt] Rewrite tst_QDeclarativeWebView::multipleWindows() to not depend on Grid internals
https://bugs.webkit.org/show_bug.cgi?id=61739

The skipped test was imported from Qt source repository, and used private headers
to peek in the QML Grid element. This patch changes the QML used to expose the
information we want to test: number of pages opened and the first page opened.

* tests/qdeclarativewebview/resources/newwindows.html:
Added <body> tags. We have no reason to not use them in the test.

* tests/qdeclarativewebview/resources/newwindows.qml:
Moved the timer out of the page component, used anchors for setting webview size,
changed the way we count pages opened. Also changed coding style a bit.

* tests/qdeclarativewebview/tst_qdeclarativewebview.cpp:
(tst_QDeclarativeWebView::multipleWindows):
We now look for properties with the information we want in the rootItem: pagesOpened and
firstPageOpened.